1. Field of the Invention
The invention relates to a structure improvement of attraction plate of electromagnetic doorlock, particularly to an attraction surface of an attraction plate as a concave-arc surface structure, forming a curved internal stress.
2. Description of the Related Art
In the access control monitoring system, the use of an electromagnetic doorlock has been very popular. The electromagnetic doorlock 10 as shown in FIG. 1 provides an electric magnet 11 mounted on a door flame 15 and an attraction plate 12 mounted on a corresponding position of a door plate 14. When the electric magnet 11 is energized to produce electromagnetic attraction and attract the attraction plate 12, the electromagnetic doorlock 10 forms in a lock state. When the electric magnet 11 is de-energized and the attraction plate 12 detaches from the electric magnet 11, the electromagnetic door lock 10 then forms in an unlock state.
The traditional attraction plate 12 as FIGS. 2 to 4 comprises an attraction surface 121 in a flat shape, and one or two positioning holes 122. The attraction plate 12 is fixed on a mounted body 13 by a screw 123 and other related accessories 124. The mounted body 13 as shown in FIG. 2 is in a box shape. With the reference to FIG. 1A, the mounted body 13 has a plurality of fixed holes 125 for fixed to the door plate 14 by a plurality screws 126, but it is not a limitation. basically, the mounted body 13 can be set for any shapes or the door plate 14 directly can be as a mounted body 13 to fix the attraction plate 12 by the screw 123 and other related accessories 124 as shown in FIG. 1B. No matter what shape of the mounted body 13 is, the combination method of the mounted body 13 and attraction plate 12 is the same, and the attraction surface 121 is a plat surface. For example, the U.S. Pat. No. 4,487,439 discloses a screw and a positioning hole, and the U.S. Pat. No. 4,652,028 discloses tow screws and two positioning holes.
This kind of structure of the attraction surface 121 is used for many years. After continuous research, the inventor found out that after the electric magnet 11 is energized, the magnetic flux density (B) is strong in the region of both ends, and the magnetic flux density (B) is weak in the middle region. Thus, as shown in FIGS. 4 and 5A, 5B, when the door plate 144 is pulled, the action force (F) is focus on the screw 123 in the middle, and the electric plate 12 is pulled by the screw 123. At this time, the middle region of the electric plate 12 is curved and deformed like a dotted line (C) as shown in FIG. 5B, and the curvature and deformation will affect the attraction effect at both sides of the electric plate 12. That is, when the region of the electric plate 12 is curved and deformed, the attraction surface 121 will departed from the electric magnet 11. The experimental result shows that when the electric magnet is subjected to 500 mA current and 12V voltage, the electric magnet with strength of 185 mm and thickness of 15 mm is easily to be pulled away from the attraction plate as the tension value is between 400 to 500 pounds. The industry claimed that the tension value can reach to 600 pounds, but the attraction plate is pulled with less than 500 pounds. Therefore, to increase the attraction force of the attraction plate 12 of the conventional electromagnetic doorlock, the current of the electric magnet 11 or the attraction area of the electric magnet and attraction plate should be increased, forming a waste of energy or increasing the materials and transportation costs. Accordingly, there is room for improvement of the structure of conventional attraction plate 12.